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Full text of "Mathematical And Physical Papers - Iii"

362         ON THE CHANGE OF RKFRANGIHILITY  OF  LIGHT.
that in the case of the beams actually observed, the suspended particles were small compared with the length of a wave of light. Observation showed that the reflected ray was polarized. Now all the appearances presented by a plane-polarized ray are symmetrical with respect to the plane of polarization. Hence we have two directions to choose between for the direction of the vibrations in ; '                         the reflected ray, namely, that of the incident ray, and a direction
'                         perpendicular to both the incident and the reflected rays.    The
former would be necessarily perpendicular to  the  directions of 1 >                         vibration in  the incident ray, and  therefore we are obliged to
i I                         choose the latter, and consequently to suppose that the vibrations
H :                         of plane-polarized light are perpendicular to the piano of polariza-
tion, since experiment shows that the plane of polarization of the
j                              the same as that of the ether itself, the other a to and fro motion along the. name
line as the former but in the opposite direction; and we may superpose these, motions an regards their effect on the ether. In the fornitT the pjtrfieli-H would bo moving with the ether, and therefore would not disturb it; an ivpiuds the latter we may think of the particles an moving to and I'm in otht-nviM' still ether, and producing therefore an ethereal disturbance emanating in all directions from the particle. This disturbance having to be transversal will evidently In: nil in a polar direction and a maximum in an equatoiml direction, varying in fact in amplitude as the nine of the polar distance, the polar line being a line through the middle of the particle drawn in the direction of the incident vibrations. The direction of propagation of the incident light and that of the line of i-ight being as in the text, if the incident light be common light, we may replace it by two independent streams, of equal intensity, polarized the one in a \vrticul and the other in a horizontal plane; and of these; the one fur which the plain* of vibration is vertical will not give rise to any ditTracte.d light entering the eye, while the other will give rise to a stream for winch the direction of vibration i.s hnri/.ontal, and which is therefore polari'/ed in such a manner that the- plane of vibration passes through the line of sight and is perpendicular to the direction of propagation of the incident light, and which therefore may be exiinguh'hed by an unnhser suitably turned; and in a similar way, UK stated in the te\t, the R'-ht entering the eye may be quenched by polari/.ing the. light before ineidenci- on the particles instead of analysing it alter diffraction. The conchr-ious of theory, which are enunciated with reference to the plane of vibration, exactly agree with the results of experiment, which are described with reference to the plane of pnlari/atuui; and to make the two lit we must suppose, the direction of vibration in polari'/ed light to be perpendicular, not parallel, to the plane of polmi/.ation.
There can be little doubt that in several cases which fell under my notice, especially in that of the glass mentioned in Art. 1K1, the particles were suOiciently fine to render the above reasoning applicable. Still more must that have been the case in the beautiful experiments of Tyndall on tin? decomposition of gases and vapours by rays of high refrangibility, who was led independently to the same conclusions as those stated in the text regarding the phenomena of polari/ation exhibited by fine particles in suspension.]